Abstract:

In 2007 the bottlenose dolphins of Doubtful Sound were reported to have been in decline for the previous 12 years. To identify and isolate potential causal factors, I began a comparative study of the resident populations in both Doubtful and Dusky Sounds, starting in 2009.

The decline in Doubtful Sound appears to have stopped. At the end of 2011 there were 61 individuals in the population (CV = 1.46%), an increase of 11% since 2007. The calving seasons in 2010 and 2011 also showed improvements in the calf survival rate (2002-2009: 0.434; 95% CI: 0.360-0.508 to 2010-2011: 0.622; 95% CI: 0.435-0.830). However, an age-structured stochastic, matrix projection model still shows a declining trend (decline in 62% of model runs). The Dusky Sound population has a higher calf survival rate (0.722; 95% CI: 0.556-0.844), but also shows a declining trend (60% of model runs).

In Doubtful Sound the dolphins were found predominantly in the warmer parts of the fiord, resulting in opposite distribution patterns in summer (favouring the innermost arms) and winter (favouring the outer fiord). In all seasons dolphins were closely associated with the fiord walls. The dolphins’ distribution was not significantly related to salinity, proximity to the power-station’s tailrace, or to the flow from the tailrace. In Dusky Sound, dolphins tended to be more dispersed. Distribution was significantly related to water temperature only in winter, but not to proximity to the fiord walls. The difference in proximity to the fiord walls may be due to differences in prey species or to differences in benthic reef geography.

In Doubtful Sound in 2002 (the year in which calf survival was believed to have dropped) there was a switch from calves predominantly being born in January. From 1992-2011 all calves born in January survived to at least one year of age. Modeling suggests that this change in birth timing may account for 47 - 100% of the drop in calf survival since 2002. Calf survival is also highly dependent on who the mother is. The seven least successful mothers have never had a calf survive to three years of age, while the seven most successful have had 19 of their 20 calves survive to three years old. Synchrony of breeding by the most successful mothers results in periods of many years when none of them are available to breed. This may explain, at least partially, the drop in survival rate around 2002. For very small populations year-to-year variation in who gives birth can have a substantial effect on calf survival rates within the population.